Federal Agencies
Digitization Guidelines Initiative

Home > Glossary > D > Display Aspect Ratio (DAR)

Term: Display Aspect Ratio (DAR)

Note:
 “Search Glossary” button searches only the glossary. Temporary note: search not enabled for two- and three-character terms; browse by alphabet.
 “Search“ button at the top right of the page searches the Web site, not the glossary.

Suggest a term

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Term: Display Aspect Ratio (DAR)

Definition:

Display Aspect Ratio (DAR) is the ratio between the width and height of the frame at the time of playback. DAR is a metadata field present in most audiovisual containers (though absent in some, including AVI) that is used by playback software and platforms to properly render the width and height of the frame proportionally so that the image is not stretched or narrowed.

DAR can be calculated with the following equation: DAR = SAR x PAR

A file can potentially display conflicting DARs. Some file containers, like AVI, may not clearly document aspect ratio, resulting in a file presentation that may stretch or adjust from the original intended image. However, even though AVI does not include a standard PAR or DAR field, there are workarounds that allow for these characteristics of the file to be documented. Some software will include PAR and DAR information in additional RIFF chunks at the end of the file; others use the XMP chunk in the file to document this information. It is also possible to use a codec such as FFv1 or others that specifically document the DAR information. For playback and transcoding, some applications allow users to force a certain desired DAR in software settings.

The three values related to aspect ratios are intertwined. SAR is an immutable value that is unchanging over time or unable to be changed. It is the size of the file. DAR is the rendering of the file, and PAR is what has changed to achieve that. In other words, PAR is adjusted in order to get the appropriate DAR from the given SAR of the file, as the pixel dimensions of a file (SAR) are immutable. If the desired DAR is different than the SAR, the PAR must be adjusted to be either larger or smaller than 1:1, which represents a perfectly square pixel. For example, a 720x480 file has a SAR of 3:2, but must be displayed with a DAR of 4:3 to be rendered properly. In this case, a PAR of 8/9 must be set in order for the file to be displayed properly, or in other worse, the pixels must be slightly taller than they are wide.

DAR is useful for normalizing the display of video content across broadcast standards at the point of playback. For example: A digitized NTSC image is stored in a frame of 720x486 pixels, while a digitized PAL image is stored in a frame size of 720x576 pixels. If either an NTSC or PAL video were to be displayed with square pixels according to their stored frame size without accommodating for DAR, both would result in the displayed image having a distorted width. The NTSC image would be wider than it should be, and the PAL image would be narrower than it should be. The inclusion of DAR in the video container ensures that the playback systems render the image as intended, rather than depending on the size of the frame that the image is stored in. In this case, the NTSC pixels are made narrower and the PAL pixels are made wider so that the resulting images are rendered as 4:3.

It is also possible to use DAR to properly display anamorphic video content. In this case, a widescreen (16:9) image is made narrower to fit inside of a standard definition frame. In the case of NTSC, a file with 720x486 pixels can be used the 16:9 image. If this file is displayed without accommodating for DAR, the image would be extremely distorted, much narrower than intended. However, the DAR information in the file (in this case, the DAR is set to 16:9) will inform the playback system that the pixels should be stretched widthwise in order to properly display the 16:9 content.

Typical values for DAR include:

  • Standard definition: 4:3
  • High definition: 16:9
  • 4:3 - Standard TV (NTSC/PAL) display and non-widescreen computer displays
  • 16:9 - Widescreen (HD) TV displays (1080p, 1080i, 720p etc), and 4K-capable displays
  • 16:10 (8:5) - Widescreen computer displays, and 4K-capable displays
  • 5:4 - Larger format computer monitors (less common)
  • 1.90:1 - Digital Cinema Initiatives standard resolution for 4K or 2K video projection
Category:
Video
Resource:
https://www.digitizationguidelines.gov/guidelines/sigpropvideo.html
Wikipedia
https://en.wikipedia.org/wiki/Display_aspect_ratio
See also:
Aspect ratio; Storage Aspect Ratio (SAR); Pixel Aspect Ratio (PAR)